Determination of the mechanical behavior physical properties of materials is necessary so that materials may be selected for use, evaluated when in use, and evaluated after use. From these determinations, decisions are made as to which materials to use, the conditions under which they can be used, and whether such materials in use can be continued to be used with safety. These types of determinations are particularly useful for determining the effects of environmental loading such as nuclear radiation on the mechanical properties of in-service materials. This invention is fully applicable to the determination of mechanical behavior of such materials but is also applicable to materials not subjected to radiation and the validity of the invention was demonstrated for materials not subjected to radiation.
The prior art includes U.S. Pat. No. 4,567,774 having the inventor, Michael P. Manahan, common with this application, and assigned to the same assignee. This earlier patent, hereinafter referred to as the "prior patent" includes the basic concepts upon which this invention is based. The disclosure of the prior patent is included herein by reference and the portions of that disclosure not specifically needed for the disclosure of this improvement invention are not included herein. However, reference to the prior patent may be helpful to the understanding hereof.
In the past, the most common procedure has been to determine the mechanical behavior of material by testing large samples that are either created more or less simultaneously or side by side with the product that is intended to be used or are cut from the same batch of material. In the determination of the mechanical behavior of solid materials and particularly metals, the practice is to make tensile, fatigue, creep, stress relaxation, fracture mode transition behavior, fracture toughness, etc. specimens; and these are then subjected to loads while measurements are taken of the force, time, displacement, energy, velocity, temperature, crack length, etc. of the specimen. Information on stress and strain, which can be thought of as normalized load and deflection, respectively, as well as other useful parameters are then obtained by simple mathematical operations. For example, in a uniaxial tensile test, the stress is determined by dividing the measured load by the specimen cross sectional area.
While this may be satisfactory in most instances, there are other circumstances such as the post-irradiation testing of materials used in nuclear reactors where samples may be unavailable in sufficient size and quantity to carry out these destructive tests during the life of the materials in use. In general, neutron irradiation space for materials investigations is limited and costly. It is, therefore, desirable to use specimens of minimum volume. Since neutron irradiation costs scale with specimen volume, miniaturized mechanical behavior testing can provide significant savings in irradiation testing costs for nuclear materials investigations. In addition, it is possible to provide mechanical behavior information which is not ordinarily obtainable due to space limitations in irradiation experiments and thus expedite alloy development investigations. Of course, miniature specimen testing is applicable to materials investigations for other nuclear technologies as well as non-nuclear technologies requiring mechanical behavior characterizations from a small volume of material. One such non-nuclear application is cutting small pieces of material from in-service components and using miniature specimens to measure the current mechanical behavior state. These data can then be used to estimate the remaining life of the in-service component.
There are four principal conceptual innovative aspects to the miniaturized fracture testing method of this invention. The first is the use of specimens that are significantly thinner and smaller than those currently in use. The second is the use of the appropriate loading configuration to either accommodate the size scale involved or better represent the actual in-service loading. The third is to modify the stress field in the specimen (caused by loading the specimen in a manner analogous to the current practice) by applying additional stressing force to the specimen so as to induce stress in the specimen in a selected orientation. This is done so as to achieve or closely approximate a desired stress state in the specimen which could not be achieved in any other way. A particular manifestation is to induce a large transverse stress normal to the conventional test loading direction which approaches a plane strain condition in a thin specimen. The rate and magnitude of the additionally induced stress is related to the rate and magnitude of the conventionally applied load on the specimen. The stress field modification in effect is used to replace the need for material thickness. The fourth is the use of the finite element method to calculate the direction and amount of additional load to be applied to achieve the desired stress state in the specimen. In a particular manifestation, the finite element code is used to determine the amount of transverse load to be applied to achieve plane strain conditions as a function of the vertically applied load for a three point bend or compact type specimen.
This invention improves upon the method of U.S. Pat. No. 4,567,774 by teaching the modifying of the stress field during fracture behavior testing with the miniature specimen. This modifying of the stress field can be done mechanically or by using a force field such as a magnetic field in order to produce stresses in preferred orientations in the material. In essence, the stress field modification replaces the need for material thickness and can minimize the need for side grooving to attain valid fracture toughness data.
Current test procedures require a minimum specimen thickness for plane strain fracture toughness testing which cannot be satisfied in many cases. This has historically served to preclude use of miniature specimens. The advantage of the present invention is that specimens which are much thinner than those currently in use can be accurately tested. This enables testing of materials removed from in-service components in cases where it is not possible to remove enough material to meet current ASTM test requirements. Another advantage of the method of the invention is that the method may allow the modifying of the stress field such that mixed mode fracture in the transition region can be avoided and/or conservatively accounted for. The invention can in many instances eliminate the need for side grooving now needed for accurate testing with many materials.
Another advantage of the present invention is that in-service stress fields can be simulated in the laboratory thereby providing data which is more representative of component performance.